Methods for forming carbon nanotube thermal pads

ABSTRACT

Methods for forming thermal pads including arrays of vertically aligned carbon nanotubes are provided. The thermal pads are formed on various substrates, including foils, thin self-supporting polished metals, semiconductor dies, heat management aids, and lead frames. The arrays are growth from a catalyst layer disposed on the substrate. Forming the array can include leaving the ends of the nanotubes unfinished, attaching a foil thereto, or coating the ends with a metal layer. The metal layer coating can then be polished to a desired smoothness. The array can be filled with a matrix material, only partially filled, or left unfilled. Where the substrate is a foil, the method can be a continuous process where foil is taken from a roll and fed through a series of formation steps. Where the substrate is a lead frame, heating can be generated by applying an current to a pad of the lead frame.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of semiconductorpackaging and more particularly to methods for forming structures thatemploy carbon nanotubes for thermal dissipation.

2. Description of the Prior Art

A carbon nanotube is a molecule composed of carbon atoms arranged in theshape of a cylinder. Carbon nanotubes are very narrow, on the order ofnanometers in diameter, but can be produced with lengths on the order ofhundreds of microns. The unique structural, mechanical, and electricalproperties of carbon nanotubes make them potentially useful inelectrical, mechanical, and electromechanical devices. In particular,carbon nanotubes possess both high electrical and thermal conductivitiesin the direction of the longitudinal axis of the cylinder. For example,thermal conductivities of individual carbon nanotubes of 3000 W/m-°K andhigher at room temperature have been reported.

The high thermal conductivity of carbon nanotubes makes them veryattractive materials for use in applications involving heat dissipation.For example, in the semiconductor industry, devices that consume largeamounts of power typically produce large amounts of heat. FollowingMoore's Law, chip integration combined with die size reduction resultsin an ever increasing need for managing power density. The heat must beefficiently dissipated to prevent these devices from overheating andfailing. Presently, such devices are coupled to large heat sinks, oftenthrough the use of a heat spreader. Additionally, to allow fordifferences in coefficients of thermal expansion between the variouscomponents and to compensate for surface irregularities, thermalinterface materials such as thermal greases are used between the heatspreader and both the device and the heat sink. However, thermal greasesare both messy and require additional packaging, such as spring clips ormounting hardware, to keep the assembly together, and thermal greaseshave relatively low thermal conductivities.

Therefore, what is needed are better methods for attaching heat sinks,sources, and spreaders that provides both mechanical integrity andimproved thermal conductivity.

SUMMARY

An exemplary method of forming a thermal pad comprises providing asubstrate having a thickness of less than 500μ and a planar surface,forming a catalyst layer over the planar surface of the substrate, andforming an array of carbon nanotubes on the catalyst layer. The array isformed such that the carbon nanotubes are generally aligned in adirection perpendicular to the planar surface. The array thus formed ischaracterized by a first end attached to the catalyst layer and a secondend opposite the first end.

The substrate is preferably thin and in some embodiments is a copperfoil or a thinned silicon wafer. The thickness of the substrate can beless than 500μ, less than 250μ, or less than 100μ. In some embodiments,an interface layer is formed on the substrate before the catalyst layeris formed. In some of these embodiments a barrier layer is formed on thesubstrate before the interface layer is formed. The catalyst layer canbe patterned so that the array forms bundles of aligned carbon nanotubeson the patterned catalyst layer. Spacers can also be provided on theplanar surface before forming the array so that the finished thermal padwill include spacers that can serve to protect the carbon nanotubes ofthe array from damage during handling and assembly.

Variations on the method include infiltrating a matrix material into thearray to fill an interstitial space between the first and second ends.Alternately, a base metal layer can be formed around the carbonnanotubes at the first end of the array such that the interstitial spacebetween the base metal layer and the second end of the array remainsunfilled. In some embodiments the interstitial space advantageouslyremains unfilled. In some further embodiments a catalyst layer is formedon both sides of the substrate and then an array of carbon nanotubes isformed on each.

The carbon nanotubes at the second end of the array can be left free inthe finished thermal pad. In some embodiments, however, a metal layer isformed on the second end of the array such that the carbon nanotubesextend at least partially into the metal layer. This metal layer canthen be polished to make it smooth. Forming this metal layer can includecoating the ends of the carbon nanotubes with a wetting layer. Thewetting layer can, in turn, be coated with a protective layer over thewetting layer. Instead of a deposited metal layer, in some embodiments ametal foil is attached to the second end of the array. Attaching themetal foil can include, in some embodiments, forming an attachment layeron the second end of the array.

In some embodiments the substrate is a foil. The foil can be supportedand handled according to several different embodiments. For example, thefoil can be supporting with a frame. In other embodiments, the foil isfed from a roll into a guide and a transport mechanism is used to movethe foil along the guide.

Another exemplary method of forming a thermal pad comprises providing alead frame having a die bonding pad, forming a catalyst layer over thedie bonding pad, and forming an array of carbon nanotubes on thecatalyst layer such that the carbon nanotubes are generally aligned in adirection perpendicular to the die bonding pad. In some embodiments,forming the array comprises heating the die bonding pad by applying acurrent to the die bonding pad. Also in some embodiments the methodfurther comprises separating the die bonding pad from the lead frame.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1-11 show cross-sectional views of thermal pads according tovarious exemplary embodiments of the invention. The orders of thelayers, from bottom to top, in each of these drawings also serve toillustrate exemplary methods of forming the thermal pads.

FIG. 12 shows a cross-sectional view of a partially completed thermalpad according to an exemplary embodiment of the invention.

FIG. 13 shows a cross-sectional view of the thermal pad of FIG. 12 afteran array of vertically aligned carbon nanotubes has been fabricatedaccording to an exemplary embodiment of the invention.

FIG. 14 shows a cross-sectional view of still another thermal padaccording to an exemplary embodiment of the invention.

FIG. 15 shows a top view of a portion of a lead frame used as asubstrate for forming a thermal pad according to an exemplary embodimentof the invention.

FIG. 16 shows a cross-sectional view of a plurality of lead framesdisposed in a tube furnace for carbon nanotube synthesis thereon,according to an exemplary embodiment of the invention.

FIG. 17 shows a cross-sectional view of the lead frames and furnace ofFIG. 16 taken along the line 17-17.

FIG. 18 shows an enlarged view of a portion of the cross-sectional viewof FIG. 17.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides methods for fabricating carbonnanotube-based thermal pads. The thermal pads are characterized by anarray of generally aligned carbon nanotubes disposed on a substrate,such as a foil, a thin metal sheet, or the surface of a component of adevice. The carbon nanotubes are disposed on the substrate such that thedirection of alignment is essentially perpendicular to the surface ofthe substrate on which the array is disposed. The alignment of thenanotubes allows the array to provide excellent thermal conduction inthe direction of alignment. Accordingly, a thermal pad between a heatsource and a heat sink provides a thermally conductive interfacetherebetween.

Some thermal pads are characterized by at least one, and in someinstances, two very smooth surfaces. A thermal pad with a sufficientlysmooth surface can adhere to another very smooth surface, such as thebackside surface of semiconductor die, much like two microscope slideswill adhere to each other. Surfaces of thermal pads, whether very smoothor not, can also be attached to an opposing surface with a metal layer,for example with solder, indium, or silver. Advantageously, some thermalpads are also characterized by a degree of flexibility and pliability.This can make it easier to work with the thermal pads in assemblyoperations and allows the thermal pads to conform to opposing surfacesthat are curved or irregular.

FIG. 1 illustrates an exemplary method of forming a thermal pad. In theexemplary method a substrate 110 with a generally planar surface 120 isinitially provided. Various examples of suitable substrates 110 aredescribed below. Next, an optional barrier layer 125 is formed on theplanar surface 120. The purpose of the barrier layer 125 is to preventdiffusion between the substrate 110 and a subsequently depositedcatalyst layer. Preventing such diffusion is desirable in thoseembodiments where the substrate 110 includes one or more elements thatcan poison the catalyst and prevent nanotube growth. Examples ofelements that are known to poison nanotube catalysis include nickel,iron, cobalt, molybdenum, and tungsten. Other substrates, such assilicon, are not known to poison nanotube catalysis and may nottherefore require the barrier layer 125. An example of a suitablebarrier layer 125 is a sputtered film of aluminum oxide with a thicknessof at least 50 Å, and more preferably 100 Å. An appropriate thicknessfor the barrier layer 125 will depend both on the permeability of theselected material to the elements to be impeded, and also on theroughness of the planar surface 120, as rougher finishes require thickerbarrier layers 125.

An optional interface layer 130 is formed over the planar surface 120,and over the barrier layer 125, if present. The interface layer 130 isprovided, where needed, to improve the subsequent catalyst layer which,in turn, provides for higher quality nanotubes characterized by higherwall crystallinities and fewer defects. In some embodiments, a singlelayer can serve as both the barrier layer 125 and the interface layer130. Again, a sputtered film of aluminum oxide with a thickness of atleast 50 Å, and more preferably 100 Å can be a suitable interface layer130. Another suitable interface layer 130 includes silicon dioxide. Itshould be noted that too thick of an interface layer 130 can lead tocracking during thermal cycling due to mismatches in coefficients ofthermal expansion between the interface layer 130 and the layer beneath.

Next, a catalyst layer 140 is formed. The catalyst layer 140 can beformed either directly on the planar surface 120 of the substrate 110,on the barrier layer 125, or on the interface layer 130, depending onthe various materials chosen for the substrate 110 and the catalystlayer 140. After the catalyst layer 140 has been formed, an array 150 ofcarbon nanotubes is formed on the catalyst layer 140. The array 150 isformed such that the carbon nanotubes are generally aligned in adirection 155 perpendicular to the planar surface 120. The array 150includes a first end 160 attached to the catalyst layer 140 and a secondend 170 opposite the first end 160. Depending on the growth conditionsand choice of catalyst, the carbon nanotubes can be single-walled ormulti-walled. The density, diameter, length, and crystallinity of thecarbon nanotubes can also be varied to suit various applications.

One general method for achieving carbon nanotube growth is to heat thecatalyst layer 140 in the presence of a carbon-bearing gas. Examples ofsuitable catalysts and process conditions are taught, for example, byErik T. Thostenson et al. in “Advances in the Science and Technology ofCarbon Nanotubes and their Composites: a Review,” Composites Science andTechnology 61 (2001) 1899-1912, and by Hongjie Dai in “Carbon Nanotubes:Opportunities and Challenges,” Surface Science 500 (2002) 218-241. Itwill be appreciated, however, that the present invention does notrequire preparing the carbon nanotubes by the catalysis methods ofeither of these references, and any method that can produce generallyaligned carbon nanotubes extending from a surface is acceptable.

FIGS. 2-5 illustrate the method set forth with respect to FIG. 1 asapplied to specific substrates. In FIG. 2 a substrate 200 representseither a thin substrate or a foil. Both a foil and a thin substrate arecharacterized by the planar surface 120 and an opposing planar surface210. In some embodiments, the planar surface 210 has an optically smoothfinish. The distinction between a foil and a thin substrate is that thethin substrate is self-supporting while the foil is not. Thus, a foilshould be secured to a supporting structure such as a pedestal or aframe during processing, while a thin substrate need not be secured.Copper and silver foils are examples of suitable foils. Suitable thinsubstrates include polished metal blanks and semiconductor wafers. Forexample, a 4″ single-crystal silicon wafer can be thinned byconventional backside thinning processes, like grinding followed bychemical mechanical polishing (CMP), to a thickness of 500μ, 300μ, 200μ,25μ or thinner.

In FIG. 3 a semiconductor die 300 manufactured from a silicon wafer, forexample, provides the substrate. In this example, the method is used togrow the array 150 on a backside 310 of the semiconductor die 300. Aheat spreader 400 used to distribute heat from a semiconductor die to aheat sink in a semiconductor package provides the substrate in FIG. 4.As shown, the array 150 can be grown by the method on either the surface410 that faces the semiconductor die, or on the surface 420 that facesthe heat sink, or both. The array 150 can also be grown on a heat sink500, as illustrated by FIG. 5.

FIGS. 6 and 7 illustrate exemplary further steps to the method ofFIG. 1. In FIG. 6 a metal layer 600 is formed on the second end 170 ofthe array 150 so that the carbon nanotubes extend partially into themetal layer 600. A suitable metal for the metal layer 600 is copper. Themetal layer 600 can be formed, for instance, by sputtering, evaporation,or electroplating. It should be noted that the metal layer 600 is notmeant to infiltrate the entire array 150 but only to encapsulate thevery ends of the carbon nanotubes and to extend a short distance abovethe second end 170. An appropriate thickness for the metal layer 600will depend on the density of carbon nanotubes in the array 150 and thevariation in their heights, but a minimum thickness for the metal layer800 is on the order of 200 Å.

In some embodiments, forming the metal layer 600 includes applying aconformal coating to the ends of the carbon nanotubes with a wettinglayer of a metal that promotes improved wetting of the metal layer 600to the carbon nanotubes. Suitable wetting layer materials includepalladium, chromium, titanium, vanadium, hafnium, niobium, tantalum,magnesium, tungsten, cobalt, zirconium, and various alloys of the listedmetals. The wetting layer can be further coated by a thin protectivelayer, such as of gold, to prevent oxidation of the wetting layer. Thewetting and protection layers may be achieved by evaporation,sputtering, or electroplating, for example. It should be noted thatthese conformal coatings merely conform to the ends of the carbonnanotubes and are not continuous films across the second end 170 of thearray 150. Wetting and protection layers are described in more detail inU.S. Non-Provisional Patent Application Number 11/107,599 filed on Apr.14, 2005 and titled “Nanotube Surface Coatings for ImprovedWettability,” incorporated herein by reference in its entirety.

As shown in FIG. 7, the metal layer 600 can be polished to increase thesmoothness of the surface. Polishing the metal layer 600 can comprisechemical mechanical polishing (CMP) which also serves to planarize thesurface. Copper is a good choice for the metal layer 600, in thoseembodiments that include CMP of the metal layer 600 in that CMP ofcopper has been refined in the semiconductor processing arts. In someembodiments, polishing the metal layer 600 continues until the secondend 170 of the array 150 is exposed, while in other embodimentspolishing is discontinued before that point is reached, as shown in FIG.7.

As shown in FIG. 8, instead of forming and polishing a metal layer 600,in other embodiments a thermal pad with a smooth surface is obtained byattaching a foil 800 to the array 150. Attaching the foil 800 caninclude forming an attachment layer 810 on the second end 170 of thearray 150 so that the carbon nanotubes extend partially into theattachment layer 810. Ideally, the attachment layer 810 is formed of alow melting point metal or eutectic alloy such as indium, tin, bismuth,or a solder such as tin-silver, tin-lead, lead-silver, gold-germanium,or tin-antimony. The attachment layer 810 may be formed by evaporation,sputtering, electroplating, or melting a thin sheet of the desiredmaterial, for example. As above, in some instances a wetting layer withor without a further protective layer can be applied as a conformalcoating on the ends of the carbon nanotubes prior to forming theattachment layer 810.

Copper and silver foils are examples of suitable foils 800. The foil 800can be joined to the attachment layer 810 by heating the foil 800 whilein contact with the attachment layer 810 to briefly melt the attachmentlayer 810 at the interface. In some embodiments, such as those in whichthe low melting point metal comprises indium, it can be advantageous tostrip the native oxide layer from the attachment layer 810 by cleaningthe attachment layer 810 with an acid such as hydrochloric acid prior toattaching the foil 800.

Each of the thermal pads shown in FIGS. 1-8 is characterized by an array150 of generally aligned carbon nanotubes with empty interstitial spacebetween the carbon nanotubes. The empty interstitial space can beadvantageous, in certain situations, as it provides the thermal padswith greater flexibility. In other embodiments, described below withreference to FIGS. 9 and 10, some or all of the interstitial space isfilled.

For example, in FIG. 9 the interstitial space is filled by a matrixmaterial 900. Examples of matrix materials include metals and polymers.The interstitial space of the array 150 can be filled by a metal, forexample, by electroplating. Injection molding can be used, for instance,to fill the interstitial space of the array 150 with a polymer such asparylene. Polymer injection molding into aligned nanotubes is taught byH. Huang, C. Liu, Y. Wu, and S. Fan in Adv. Mater. 2005, 17, 1652-1656.Both metal and polymers can be useful to provide additional structuralsupport, while metals also provide some additional thermal conductivity.

FIG. 10 shows the interstitial space of the array 150 partially filledwith a base metal layer 1000 that surrounds the carbon nanotubes at thefirst end 160 of the array 150 but otherwise leaves the interstitialspace empty. The base metal layer 1000 can be formed of a metal such ascopper by electroplating with the catalyst layer 140 serving as anelectrode. The base metal layer 1000, like the matrix material 900, isadvantageous for further securing the array 150 to the catalyst layer140. The base metal layer 1000 both provides this advantage while stillleaving much of the interstitial space empty for greater flexibility ofthe thermal pad. It should be understood that the matrix material 900,or base metal layer 1000, can be applied to any of the embodimentstaught with respect to FIGS. 1-8.

FIG. 11 illustrates yet another variation on the method of forming athermal pad. In this example, the catalyst layer 140 is patterned, priorto forming the array 150, so that the carbon nanotubes of the array 150grow in columns or bundles 1100. The catalyst layer 140 can bepatterned, for example, by conventional masking techniques known to thesemiconductor processing arts. Patterning the catalyst layer 140 toproduce the bundles 1100 can be useful for those thermal pads that donot have a top layer such as metal layer 600 or foil 800. When thesecond end 170 of the array 150 of such a thermal pad is joined to asurface, the taller bundles 1100, because of the spaces between thebundles 1100, are able to bend until the shorter bundles 1100 alsocontact the surface. In a similar manner, bundles 100 can be beneficialto thermal pads even with a top layer to allow the top layer to deformto match the contour of a mating surface.

It should be noted that a continuous catalyst layer 140, as shown forexample in FIG. 1, can be patterned to include a varying composition,thickness, or density of catalyst particles. Examples of such patternedcatalyst layers are described in more detail in U.S. Non-ProvisionalPatent Application Number 11/124,005 filed on May 6, 2005 and titled“Growth of Carbon Nanotubes to Join Surfaces,” incorporated herein byreference in its entirety. Providing such patterning can be advantageousto vary aspects of the carbon nanotubes within the array 150 as afunction of location. For example, where the thermal pad is intended toprovide an interface with a backside of a semiconductor die with a knowncurvature, such as a convex shape, the heights of the carbon nanotubescan be varied from shorter at the center of the array 150 to longer atthe edges. Likewise, a greater density of carbon nanotubes can be grownin areas of the array 150 in order to match the greater density to hotspots on the heat source.

FIGS. 12 and 13 illustrate still another variation on the method offorming a thermal pad. In this example, spacers 1200 are placed over theplanar surface 120 of the substrate 110 before the array 150 is formed.In some embodiments, the spacers 1200 are placed on the catalyst layer140 as shown in FIG. 12. Subsequently, the array 150 is formed, as shownin FIG. 13. Preferably, the array 150 is grown until a height of thearray 150 exceeds a height of the spacers 1200. A thermal pad includingspacers 1200 can be advantageous during assembly of the thermal padwithin a device, package, or other structure. Not only can the spacers1200 provide an appropriate spacing between two objects such as a heatsource and a heat sink, but the spacers 1200 can also prevent damage tothe carbon nanotubes of the array 150 by limiting the extent to whichthe carbon nanotubes can be deformed during handling and assembly.Suitable spacers are described in more detail in U.S. Non-ProvisionalPatent Application Number 11/124,005 noted above.

FIG. 14 illustrates that the method can also be used to provide an array150 on both surfaces of a foil 800. In these embodiments the method canbe applied to one surface and then the other, or to both surfacessimultaneously. Additionally, each of the several layers 125, 130, 140can be formed first on one surface and then on the other, while the twoarrays 150 are then grown simultaneously. A thermal pad formed by thismethod advantageously includes approximately twice the thickness ofcarbon nanotubes after an equivalent processing time.

As the foil 800 requires some form of support, a frame (not shown) canbe used, for example, to support the foil 800 having a catalyst layer140 on both surfaces within a reaction chamber while arrays 150 ofcarbon nanotubes are synthesized on both surfaces. Similarly, as notedabove in connection with FIG. 4, arrays 150 can be formed on multiplesurfaces of other substrates such as the heat spreader 400. In someembodiments multiple arrays 150 on a substrate are formed sequentiallywhile in other embodiments the arrays 150 are formed simultaneously.

Another variation on the method performs the steps in a continuousfashion on the foil 800. In these embodiments the foil 800 is initiallywound on a spool. One end of the foil 800 is fed into a guide thatprovides support to the foil 800 while a transport mechanism carries thefoil 800 through a series of sequential processes to form the variouslayers 125, 130, 140, the array 150, and any subsequent layers such asattachment layer 810. This variation can be used to form the array 150on only one side of the foil 800 or both sides, as in FIG. 14. The foil800, once fully processed, can be sectioned to form individual thermalpads or wound onto another spool. In other embodiments, only the layers125, 130, 140 are formed on the foil 800 in the described manner, thenthe foil 800 is cut into sections or coupons, and these sections orcoupons are individually or batch processed to form arrays 150 thereon.

FIG. 15 shows yet another alternate substrate for carrying out themethod. In FIG. 15 a lead frame 1500 serves as the substrate. The leadframe 1500 includes a die bonding pad 1510 and support fingers 1520 thatattach the die bonding pad 1510 to the remainder of the lead frame 1500which can include a plurality of other identical die bonding pads 1510.Thus, an array 150 can be formed on each pad 1510 of the lead frame 1500by the method described above. In some embodiments, the lead frame 1500is made of oxygen free high conductivity copper. A suitable thicknessfor a lead frame 1500 is about 250μ, though thinner and thicker ones canbe used. After processing to form the array 150, the die bonding pad1510 with the array 150 thereon can be separated from the remainder ofthe lead frame 1500 by detaching the pad 1510 from the support fingers1520. In other embodiments, the die bonding pad 1510 is supported on apedestal during processing and the pedestal heats the die bonding pad1510 from beneath, for example, by inductive heating. It will beappreciated that these same heating techniques can also be applied toother embodiments described herein.

Various steps involved in forming the layers on the die bonding pads1510 can require elevated temperatures. In some embodiments, an electriccurrent, on the order of tens of amps, is applied across the die bondingpad 1510 in order to heat the die bonding pad 1510 during variousdeposition steps such as forming the array 150. The electric current canbe applied to the die bonding pad 1510 through probes that contacteither ends of die bonding pad 1510 or close by on the support fingers1520.

FIGS. 16-18 illustrate an exemplary arrangement of a plurality of leadframes 1500 within a furnace 1600 for chemical vapor processing (CVD) toproduce arrays 150. FIG. 16 shows a cross-section through the furnace1600, FIG. 17 shows a cross-sectional view of the furnace 1600 takenalong the line 17-17 in FIG. 16, and FIG. 18 shows an enlarged view of aportion of FIG. 17 to show the lead frames supported in a boat 1800. Anexemplary furnace 1600 is a 5-inch thermal CVD system configured suchthat a carbon-containing gas can enter from one end of the furnace 1600,react to form the arrays 150 on the lead frames 1500, and exit theopposite end of the furnace 1600.

FIGS. 1-14 also represent different embodiments of finished thermalpads. The methods described herein are suitable to produce thermal padswith surface areas ranging from about 1 mm×1 mm, or less, to over 6″×6″.Arrays 150 of nanotubes can have thicknesses ranging from a few micronsto over 1 mm. In particular, the thickness of the arrays 150 can bebetween 0.1 mm and 2 mm. Some thermal pads are characterized by a secondend 170 with exposed nanotubes. Other thermal pads are characterized bya capped second end 170 where the capping is achieved with either anattached thin substrate 200 or foil 800, or a metal layer 600 that iseither unfinished, polished, or polished and planarized. Additionally,any of these thermal pads can include carbon nanotubes grown in bundles1100, and any can include spacers 1200.

Any of these thermal pads can include a matrix material 900 that fillsthe interstitial space between the ends 160, 170 of the array 150.Similarly, any can include a base metal layer 1000 that only partiallyfills the interstitial space of the array 150 around the carbonnanotubes at the first end 160. Also, the interstitial space of any ofthese thermal pads can be left empty. As noted above, keeping theinterstitial space empty improves flexibility. It should also be notedthat keeping the interstitial space empty also improves compliance ofthe thermal pad to differential thermal expansion between opposingsurfaces of two objects. The flexibility and pliability of some thermalpads allows them to be attached to curved surfaces in addition togenerally flat surfaces.

Some thermal pads are fixedly attached to inflexible substrates, such asheat spreaders, where the second end 170 of the array 150 is meant to beattached to the surface of some other object. Other such thermal padsare free-standing components meant to be disposed between the opposingsurfaces of a heat source and a heat sink. With the exception of thethermal pad shown in FIG. 14, these thermal pads are characterized by afoil or thin substrate attached to the first end 160. The thermal pad ofFIG. 14 is characterized by a foil 800 between two arrays 150 where eacharray 150 presents a second end 170 with exposed nanotubes.

A thermal pad having a second end 170 with exposed nanotubes can bejoined to a surface of an object with a low melting point metal oreutectic alloy or a solder. One advantage of this method of joining thethermal pad to the surface is that neither the surface nor the secondend 170 needs to be particularly smooth. Irregularities in either arefilled by the low melting point metal, eutectic alloy, or solder.Reworking can be easily accomplished by low temperature heating.

A thermal pad having a second end 170 with exposed nanotubes can also bejoined to a surface of an object simply by pressing the two together,known herein as “dry-pressing.” Dry pressing can be accomplished with orwithout the addition of pressure and heat. Modest elevated temperatures(e.g. 200-300° C.) and pressures (e.g., 10 to 100 psi) can be used. Insome embodiments, sufficient heat is applied to soften or melt thesurface of the object, for example, the copper surface of a heat sink,so that the ends of the carbon nanotubes push into the surface. In theseembodiments it can be advantageous to perform the dry-pressing in anon-oxidizing environment such as an oxygen-free atmosphere.Dry-pressing can also comprise making the ends of the carbon nanotubestemporarily reactive. Here, plasma etching can be used, for example, toetch away amorphous carbon and/or any catalyst materials. Plasma etchingcan also create reactive dangling bonds on the exposed ends of thecarbon nanotubes that can form bonds with the opposing surface. Drypressing can also comprise anodic bonding, where a strong electric fieldpulls ions from the interface to create a strong bond.

Either end of a thermal pad that comprises a thin substrate 200, a foil800, an unfinished metal layer 600 (FIG. 6), or a polished metal layer600 (FIG. 7) can be joined to a surface of another object in severalways. One method is to join the surface of the object with a metalhaving a melting point below the melting points of the object and theopposing surface of the thermal pad. For example, silver can be used tojoin a copper heat spreader with a palladium metal layer 600. Lowermelting point metals such as indium and solder can also be used. In someembodiments the low melting point metal is cleaned with an acid such ashydrochloric acid to remove the native oxide. In the case of a thinsubstrate 200 comprising silicon, the silicon surface can be metallizedwith titanium and then silver to bond well to the low melting pointmetal.

In other instances, where both the surface of the object and the exposedsurface of the thin substrate or foil are very smooth, the two can beheld together by van der Waals attractions. In still other instances,both the surface of the object and the exposed surface of the thinsubstrate or foil are compositionally the same or very similar, forexample where both comprise silicon. In this example, Si—Si bonds canspontaneously form between the two surfaces.

In the foregoing specification, the invention is described withreference to specific embodiments thereof, but those skilled in the artwill recognize that the invention is not limited thereto. Variousfeatures and aspects of the above-described invention may be usedindividually or jointly. Further, the invention can be utilized in anynumber of environments and applications beyond those described hereinwithout departing from the broader spirit and scope of thespecification. The specification and drawings are, accordingly, to beregarded as illustrative rather than restrictive. It will be recognizedthat the terms “comprising,” “including,” and “having,” as used herein,are specifically intended to be read as open-ended terms of art.

1. A method of forming a thermal pad comprising: providing a substratehaving a thickness of less than 500μ and a planar surface; forming acatalyst layer over the planar surface of the substrate; and forming anarray of carbon nanotubes on the catalyst layer such that the carbonnanotubes are generally aligned in a direction perpendicular to theplanar surface, the array characterized by a first end attached to thecatalyst layer and a second end opposite the first end.
 2. The method ofclaim 1 wherein the substrate includes copper.
 3. The method of claim 1wherein the substrate includes silicon.
 4. The method of claim 1 whereinproviding the substrate includes supporting the substrate with a frame.5. The method of claim 1 wherein providing the substrate includesfeeding a foil from a roll into a guide and using a transport mechanismto move the foil along the guide.
 6. The method of claim 1 furthercomprising forming an interface layer on the substrate before formingthe catalyst layer.
 7. The method of claim 6 wherein forming theinterface layer includes depositing aluminum oxide.
 8. The method ofclaim 6 further comprising forming a barrier layer on the substratebefore forming the interface layer.
 9. The method of claim 1 furthercomprising infiltrating a matrix material into the array to fill aninterstitial space thereof between the first and second ends.
 10. Themethod of claim 9 wherein infiltrating the matrix material includesinjection molding a polymer to fill the interstitial space.
 11. Themethod of claim 1 further comprising forming a base metal layer aroundthe carbon nanotubes at the first end of the array such that aninterstitial space of the array between the base metal layer and thesecond end of the array remains unfilled.
 12. The method of claim 1further comprising forming a metal layer on the second end of the array,wherein the carbon nanotubes extend at least partially into the metallayer.
 13. The method of claim 12 further comprising polishing the metallayer.
 14. The method of claim 12 wherein forming the metal layerincludes coating the ends of the carbon nanotubes at the second end ofthe array with a wetting layer.
 15. The method of claim 14 furthercomprising coating the ends of the carbon nanotubes with a protectivelayer over the wetting layer.
 16. The method of claim 1 furthercomprising attaching a metal foil to the second end of the array. 17.The method of claim 16 wherein attaching the metal foil includes formingan attachment layer on the second end of the array.
 18. The method ofclaim 1 wherein forming the catalyst layer includes patterning thecatalyst layer to form a patterned catalyst layer, and wherein formingthe array includes forming bundles of aligned carbon nanotubes on thepatterned catalyst layer.
 19. The method of claim 1 further comprisingproviding a spacer on the planar surface before forming the array. 20.The method of claim 1 further comprising forming a second catalyst layeron a second planar surface of the substrate; and forming a second arrayof carbon nanotubes on the second catalyst layer such that the carbonnanotubes are generally aligned in a direction perpendicular to thesecond planar surface.
 21. A method of forming a thermal pad comprising:providing a lead frame having a die bonding pad; forming a catalystlayer over the die bonding pad; and forming an array of carbon nanotubeson the catalyst layer such that the carbon nanotubes are generallyaligned in a direction perpendicular to the die bonding pad.
 22. Themethod of claim 21 further wherein forming the array comprises heatingthe die bonding pad by applying a current thereto.
 23. The method ofclaim 21 further comprising separating the die bonding pad from the leadframe.